Let the number be 'm' & 'n'
mn = -84
m + n = 25
Algebraically rearrgange
m = -84/n
m = 25 - n
Substitute for 'm'
-84/n = 25 - n
Multiply through by 'n'
-84 = 25n - n^2
n^2 - 25n - 84 = 0
It is now in Quadratic Form . You can either try and factor or apply the Quadratic Eq'n.
n = { - - 25 +/- sqrt[(-25)^2 - 4(1)(-84)]} / 2(1)
n = { 25 +/- sqrt[ 625 + 336]} / 2
n = { 25 +/- sqrt[961]} / 2
n = {25 +/- 31}/2
n = 56/2 or -6/2
n = 28 or -3
Hence m = -3 or 28
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-3 and 28
x*y=-84 x+y=25 (What's known)
x=-84/y x=25-y (Solve for x)
-84/y=25-y (Set them equal using the solved x's)
-84=25y-y^2 (Simplify)
y^2-25y-84=0 (Solve for 0)
(y-28)(y+3)=0 (Factor)
y=28|y=-3 (Solve for solutions)
Factors of 84:
1|84
2|42
3|28
4|21
6|14